In nonlinear interactions involving harmonic generation, it is desirable to employ non-critical phase matching (NCPM). One challenge associated with harmonic generation processes involves the beam walk-off-effect. The walk-off-effect is known to be detrimental to the nonlinear conversion process because it limits the effective interaction length between the beams generated within the laser cavity during the harmonic generation process and reduces the overall efficiency of conversion.
In the case of second harmonic generation (SHG), the desire to eliminate walk-off between the ordinary and extraordinary rays has increasingly led to the use of the nonlinear crystal lithium triborate (LBO). Conversion efficiencies in excess of 50% have been demonstrated using Type-I phase matching (PM) with this crystal, which can be temperature tuned at around 150° C. to maintain the non-critical phase-matching conditions. In recent years, LBO has largely replaced KTP as the crystal of choice to frequency double radiation from diode pumped Nd-lasers such as Nd:YAG and Nd:YVO4 in configurations both external and internal to the cavity due to its low absorption at both fundamental (1.064 μm for both Nd:YAG and Nd:YVO4) and the second harmonic (SH) at 532 nm. In addition, this material has a high damage threshold and resistance to grey tracking relative to other materials.
There are, however, situations where NCPM LBO cannot be used advantageously. For example, heretofore LBO was not used as a NCPM tripler for third harmonic generation (THG) of 1064 nm beams. Further, alternate materials, including the newly developed GdYCOB fail to meet cost and lifetime requirements essential to practical laser systems. Thus, most existing diode-pumped laser systems designed to produce UV light at high repetition rates utilize tripler crystals such as LBO and BBO in CPM arrangements. While these systems were somewhat successful as third harmonic generators, a number of shortcomings were identified. For example, the walk-off between the ordinary and extraordinary rays in the CPM tripler crystal, the separation between the fundamental and second harmonic beams which progressively increases as the beams propagate through the crystal reduces the THG conversion efficiency.
In response thereto, a number of approaches have been devised to compensate for the walk-off effect in THG processes. For example, Pieterse et al. in U.S. Pat. No. 5,835,513 taught the use of two CPM crystals in a THG process wherein the walk-off generated by a suitably orientated tripler second crystal is offset by the walk-off in the first doubler crystal. Pieterse further taught that an optical element may be positioned intermediate to the two crystals so as to modify the walk-off angle produced by the doubler thereby maximizing the conversion efficiency into the UV. While this approach was somewhat successful in reducing walk-off effects, a number of shortcoming were identified. For example, complete compensation for walk-off effects was often difficult to achieve. Further, the cost and complexity of the laser system was increased.
An alternate approach disclosed in U.S. Pat. No. 5,136,597, issued to Nightingale proposed to use a non-collinear arrangement for the purpose of compensating for walk-off in a SHG process. Specifically, Nightingale taught that a doubler crystal can be cut at a specific angle to thereby alter the direction of incidence of the fundamental beam from normal to the crystal's face.
While useful in some applications, some shortcomings associated with the method Nightingale disclosed have been identified. More specifically, Nightingale failed to appreciate the complications that may arise when attempting to compensate walk-off angles of two incident extraordinary beams, a situation which is sometimes referred to as Type-III PM.
In light of the foregoing, there is an ongoing need for a method of methods of generating harmonics wherein the walk-off of both the fundamental and harmonic beams can be optimally compensated to thereby improve the harmonic generation efficiency in practical laser systems